1. Field of the Invention
This invention relates to improvements in manufacturing and operating techniques for mass data storage devices, and the like, and more particularly to improvements in methods and apparatuses for reducing the effects of thermal asperities of an MR read head, or the like, in a signal read back from a rotating magnetic disk of a mass data storage device, or the like.
2. Relevant Background
Mass data storage devices include tape drives, as well as hard disk drives that have one or more spinning magnetic platters or disks onto which data is recorded for storage and subsequent retrieval. Hard disk drives may be used in many applications, including personal computers, set top boxes, audio, video, or television applications, or some mix thereof. Many applications are still being developed. Applications for hard disk drives are increasing in number, and are expected to further increase in the future.
One class of mass data storage devices to which the present invention has particular applicability is hard disk drive systems. A hard disk drive system typically includes a rotating magnetic disk on which information is recorded. A read transducer is movably supported adjacent the magnetic disk for reading the prerecorded information from the disk. The read transducer typically flies above the surface of the disk, being supported by an xe2x80x9cair bearingxe2x80x9d that is created by the spinning disk, so that the transducer does not touch the surface of the disk in normal operation.
Recently, magnetoresistive (MR) heads have been gaining wide popularity for use as the read transducer. The term xe2x80x9cmagnetoresistancexe2x80x9d refers to the change in resistivity of the materials of the head in the presence of the magnetic field induced in the head by the magnetic domains recorded on the disk. The introduction of MR heads into disk drives has significantly increased the a real density. However, accompanying the MR head is the problem of thermal asperity disturbances, which can cause unrecoverable errors.
A thermal asperity disturbance results when a metal particle, disk defect, or the like nearly or actually collides with the MR head, momentarily raising the temperature of the sensor. The heat conducted into the MR sensor subsequently diffuses slowly. This rapid rise in temperature changes the MR resistance and results in a voltage transient. When superimposed on the normal read back signal, the resultant shape shows a rapid rise in voltage followed by an exponential-like decay.
Similarly, if a dip in the disk exists that has the effect of increasing the air-bearing gap between the MR head and disk surface, a decrease in the cooling effect may occur in the MR head. The resulting change in resistance of the MR head material is the same as that produced by the head heating effects described above.
If the disk surface or an asperity momentarily comes closer to the MR read element without touching it an increase in the cooling effect may occur in the MR head. The resulting change in resistance of the MR head material is the same as that produced by the head heating effects described above, but in the opposite direction.
Heating and cooling effects due to the texture of the medium surface are a class of thermal asperity, sometimes known as a xe2x80x9cbaseline wanderxe2x80x9d type of event. Herein both heating and cooling type events are referred to as xe2x80x9cthermal asperitiesxe2x80x9d.
Many efforts have been directed to reducing the effects of thermal asperities. Physically, efforts have been made to reduce the flash temperature that results from a collision between the head and the disk or a defect thereon. The flash temperature can be reduced by reducing the dynamic friction, the slider dimensions, and the interaction height. The latter requires smoother disks, fewer xe2x80x9cglide escapesxe2x80x9d, lower particle count and less contamination and debris. The industry trends of lowering the flying heights and increasing the slider-disk velocities however more than offset any improvements that can be expected from these countermeasures.
Other physical measures have been taken, as well, including designing the heads to have a high magnetic sensitivity, a low effective temperature coefficient, and a wide track width. Some proposals even include using a second, dummy sensor away from the air-bearing surface of the main sensor to provide a reference against which the output of the main sensor can be compared. Differentially sensed dual stripe heads were also used to partially cancel the thermal asperity effects. Other physical measures have been taken, as well.
In addition to the physical measures, electronic compensation measures in the read channel of the device have also been taken. Both xe2x80x9con-the-flyxe2x80x9d and xe2x80x9cre-tryxe2x80x9d types of counter measures have been advanced to lessen the impact of the thermal asperity effects. The on-the-fly methods in include xe2x80x9ccloakingxe2x80x9d methods in which the analog channel front-end processes the thermal asperity events such that they become invisible to the rest of the channel. The re-try methods include recovery steps that are implemented at the system level as part of a data recovery procedure.
In any event, the detection that a thermal asperity event has occurred is of importance. Many techniques for such detection have been advanced. In one technique, a flag is generated that signals that a thermal asperity event is occurring. In another technique, onset/magnitude detectors are used, sometimes in combination with a circuit or signal processor that subtracts predetermined electronically generated thermal asperity waveforms from the data signal.
One type of detector that has been used is a window detector, which detects the onset of a thermal asperity event as indicated by a rising edge in the output at the moment at which the input signal rises above or falls below a certain threshold. Another type detector is the envelope zero-crossing detector, which compares the positive signal envelope, the baseline, and the negative envelope. Envelope detectors rapidly follow a fast rising signal, but discharge slowly when following a falling signal.
To recover from the occurrence of a thermal asperity event, waveform-recovering detectors have been used in direct electronic restoration schemes that subtract the recovered event from the incoming data signal. Such event detectors must be fast and accurate.
Regardless of the manner by which the thermal asperity event is detected, however, the information that is obtained by previous techniques has been used to map the disk of the drive, and more particularly, to map areas of the drive that are affected by the thermal asperities thereon.
However, such information has been generally insufficient to map the drive in relation to the severity of the thermal asperity effects produced in the system. Such detailed map, according to the present invention, can be used, for example, to characterize the drive to particularly identify unusable areas thereof, and can, more particularly, be used to characterize the individual thermal asperities that occur during use of the disk so that regions of the disk can be evaluated depending upon the nature of the thermal asperities that occur within various regions of the disk.
What is needed, therefore, is a method for testing and mapping a disk drive surface for the existence of thermal asperity incident creating structures, imperfections, debris, or the like, to thereby enable thermal asperity abatement settings to be selectively tailored or adjusted to individualize the compensation needed for each particular identified thermal asperity causing structures, imperfections, debris, or the like. Furthermore, this characterization information can be fed back into the disk manufacturing and handling processes to help fine-tune, refine, improve, and control these processes.
In light of the above, therefore, it is an object of the invention to provide a method for testing and mapping a disk drive surface for the existence of thermal asperity incident creating structures, imperfections, debris, or the like.
It is another object of the invention to provide a method of the type described that enables thermal asperity abatement settings to be selectively tailored or adjusted to individualize the compensation needed for each particular identified thermal asperity causing structure, imperfection, debris, or the like.
It is yet another object of the invention to provide a method of the type described that enables a greater surface area of the disk of a disk drive to be used through the identification and mapping of areas of the disk that produce thermal asperity effects in an MR head that are within the capabilities of the detector of the drive to correct.
One of the advantages realized by the invention is the ability to characterize the thermal asperity causing structures enables the development of information that can be fed back into the disk manufacturing and handling processes to help fine-tune, refine, improve, and control these processes.
These and other objects, features and advantages of the invention will be apparent to those skilled in the art from the following detailed description of the invention, when read in conjunction with the accompanying drawings and appended claims.
According to a broad aspect of the invention, a method is presented for operating a drive of a hard disk drive, or the like, of the type that uses a head that flies over a surface of a rotating magnetic disk to at least read information recorded on the disk. The head is of the type that is affected by thermal asperity effects, such as a magneto-resistive (MR) head, or the like. The method includes determining an energy level in a prerecorded constant signal that is produced on readback that exists above a predetermined threshold produced by the head in reading an area of the disk that causes a thermal asperity incident in the head. (A continuous signal means a single frequency pattern or sequence written to the disk.) Based on the energy level determined, an unusable area of the disk may be determined.
According to another broad aspect of the invention, a method is presented for characterizing a magnetic disk to be read by a magneto-resistive type head in proximity thereto. The method includes writing a continuous signal onto the disk, and subsequently reading back the signal written onto the disk using the magneto-resistive type head, or the like. The read back signal is compared to a threshold value, and areas of the disk at which energy contained in the read back signal occurs above the threshold value are mapped.
According to still another broad aspect of the invention, apparatus for characterizing a disk of a mass data storage device, or the like, is presented. The apparatus is of the type that has a head that flies over a surface of a rotating magnetic disk, or the like, to at least read information, which has been recorded on the disk. The head is of the type that is affected by thermal asperity effects, such as an MR head, or the like. A threshold detector is connected to receive a signal from the head to determine when the signal from the head exceeds a predetermined threshold. An energy detector determines an energy level in the signal during a time when the signal exceeds the threshold due to the thermal asperity incident in the head.
According to yet another broad aspect of the invention, an apparatus for characterizing a magnetic disk of the type that contains data to be read by a magneto-resistive type head in proximity thereto is presented. The apparatus has a writer for writing a continuous signal onto the disk and a comparator for comparing a signal read back from the disk using the magneto-resistive type head with a threshold value. An energy determining circuit determines an energy level contained in the read back signal during a time that the read back signal exceeds the threshold value and for mapping areas of the disk at which the energy level contained in the read back signal exceeds the threshold value. A register contains the mapped areas of the disk.